The time and space available for fuel air mixing in an internal combustion engine is limited, and "Homogeneous charge" engines do not burn really homogeneous mixtures. The incompleteness of mixing, and the unsteadiness of air fuel ratio delivery to the cylinders under transient response, degrade engine performance significantly from what would be possible with quicker transient response and more homogeneous mixing. Because the physics of mixing processes is complicated, and because mixing states are difficult to measure experimentally, the great importance of mixing in engines is not widely understood. It is the purpose of the present invention to use structured turbulent flows to mix fuel and air in an organized way involving much higher mixing rates than have previously been possible. The device is characterized by excellent transient response and much more homogeneous fuel air mixtures than have previously been practically available. To understand the technical problems which the current invention has solved, a discussion of conventional engine mixing processes is appropriate.
It is characteristic of present day carburetor and intake manifold systems that the fuel which passes the carburetor throttle is rapidly separated from the air and deposited downstream of the carburetor at the first turn. At this first turn is generally located an exhaust-heated hot spot. However, only a part of the fuel can evaporate on this hot spot surface. The rest of the fuel is not evaporated at the hot spot and deposits on manifold walls downstream. Once this fuel is deposited, it proceeds to the individual cylinders relatively slowly and somewhat unevenly.
Intake manifolds are carefully designed with contours on the manifold floor to try to distribute the liquid fuel evenly between cylinders. Even so, it is usually impossible to get very tight cylinder-to-cylinder distribution over all of the relevant speeds, under steady-state conditions of engine speed and load. Fuel-air proportioning is worse under transient conditions. The air velocity in manifold passages can be a substantial fraction of the speed of sound, but the fuel liquid film velocity is generally less than a tenth of the air velocity. Consequently, if an element of fuel and an element of air both leave the carburetor throttle at the same time, the fuel takes much longer to reach the cylinders than the air. On accelerations the mixture delivered to the cylinders therefore tends to shift lean. Acceleration enrichment arrangements must therefore be employed. The greater the acceleration enrichment, the greater the emission penalty involved.
The two-phase flow situation in a conventional intake manifold is quite involved. It is practically impossible to get the transient characteristics and cylinder-to-cylinder distribution characteristics that are desirable, even with very laborious development work in each intake manifold design because of the two phase flow relationships. The inability of conventional carburetor-intake manifold systems to perform well with respect to cylinder-to-cylinder and transient response characteristics has been the main motivation for the development of very expensive multi-cylinder fuel injection systems.
The need for fast transient response and tight cylinder-to-cylinder distribution becomes greater as emission specifications become more stringent and as fuel consumption becomes a more and more important issue. In engines which employ a three-way catalyst system to control emissions the "window" of satisfactory operation is of the order of + or--0.1 air/fuel ratio. With the 3-way catalyst system the penalty for slow transient response and inadequate cylinder-to-cylinder distribution can be drastic increases in nitric oxide production. Moreover, with emission control hardware, fuel and air are no longer the only two fluids to be mixed; in addition, it may be necessary to secure even distribution of exhaust gas recirculation from cylinder-to-cylinder.
Another approach to emission control is lean and dilute combustion. Operation with very lean (or EGR dilute) mixtures results in very low NO.sub.x emissions, and is advantageous from the point of view of the thermodynamic cycle. If fast and consistent combustion of lean or dilute mixtures is possible, significant improvements in fuel economy are achievable simultaneously with excellent NO.sub.x control. It will be shown in the detailed discussion that the level of enleanment or dilution permissible with good combustion and good drivability is very sensitive to the details of the mixing state of the fuel air EGR mixture. As cylinder-to-cylinder and microscale mixing statistics become tighter, leaner and more dilute mixtures can be efficiently burned. Therefore, the excellent mixing of the present invention widens the air fuel ratio limits of satisfactory engine combustion and permits significant improvement in emissions and fuel economy with dilute mixtures. Experimental data with the mixing vortex have been obtained which indicate that it will be possible to achieve the required NO.sub.x control with much improved fuel consumption with this lean combustion approach, without any necessity for 3-way catalysis.
The mixing state inside the cylinders and the cylinder-to-cylinder variation delivered to the engine is controllable by the state of mixing upstream of the manifold runners themselves. For some time it has been known that an intake manifold which receives a homogeneous mixture of fuel and air will distribute a homogeneous air fuel mixture to its individual cylinders. This is reasonable, since the flow of mixed gases through a passage cannot well be expected to unmix the gases. Condensation rates of fuel from a homogeneous vaporized mixture are generally quite low, even if the manifold is below the equilibrium air distillation EAD, temperature. Also the intake manifold passages quickly warm above the condensation temperature of the fuel-air mixture under normal engine operating conditions. Therefore, if the fuel from the carburetor or other fuel-air metering system can be homogeneously mixed with the air from the carburetor (and with EGR, Exhaust Gas Recirculation) prior to delivery to the intake manifold, design of the intake manifold can be very much simplified.
Designing a manifold for low flow resistance and good mass-flow distribution from cylinder-to-cylinder is a much easier problem if the manifold only handles pre-mixed vapors. The difficulty of designing manifolds presently comes because they are asked to be at once mixing devices, evaporators, and flow channels for both liquid fuel on walls and for the much higher velocity air stream in the flow channels.
It is therefore desirable to design a system where the fuel is evaporated and homogeneously mixed with the air and with any EGR prior to introduction of the mixture into the intake manifold per se. Advances in fluid mechanical knowledge based on the fluid mechanical field of "fluidics," and conceptual advances with respect to the interaction between flow structure, turbulence, and mixing largely made by R. Showalter, have made design of such a mixing system on the basis of calculable physical effects possible. To understand how this can be so, it is necessary to describe in a little detail the physical processes which must occur in order to evaporate fuel into the air and homogeneously mix the fuel with the air.
First of all, it is useful to consider the process of fuel evaporation. Research has shown that, at least so far, the only practical way to achieve droplet sizes sufficiently small so as to be in stable aerosol suspension under the turbulent flow conditions in an intake manifold (below about 1 micron) is to have these droplets form by condensation from the vapor. Complete mixing requires either complete evaporation or stable aerosol droplets. Even if the mixture out of the mixer involves stable liquid droplets, rather than completely evaporated fuel, the fuel must have been originally evaporated and then recondensed if it is to be in stable aerosol sized droplets.
Fuel vapor will evaporate into a surrounding gas only if the vapor pressure of the fuel at the liquid surface exceeds the partial pressure of the fuel vapor in the adjacent mixture. It is important to visualize the scale on which the evaporation process happens. At the liquid interface, molecules of fuel are evaporating and condensing continuously, and the number of molecules is sufficiently large that the statistical process is governed by exact physical laws. The volumes of vapor adjacent the liquid surface determine whether the liquid will evaporate or not. These vapor volumes are tiny, and are of the order of relatively few mean-free paths on a side (volumes of the order of cubic microns). Because vapor pressures in volumes so small are determinants of the evaporation process, evaporation is very closely coupled with mixing processes. Unless the rate at which fuel vapor leaves the vicinity of the liquid surface and diffuses into the air-EGR mixture is large, evaporation rates will be slow, even though the mixture is on gross volumes well away from saturation conditions. The elements of gas relevant to the evaporation process can be saturated, at the same time that the total volume inside the mixer is far away from saturation.
Evaporation is produced by the interaction of mixing at all scales, the gross vapor pressure of fuel in the air, and the vapor pressure of the liquid fuel surfaces. The vapor pressure of the liquid fuel is clearly related to heat transfer issues. To evaporate the fuel requires a supply of the fuel's heat of vaporization. Heat transfer also matters because liquid vapor pressure is a strongly increasing function of the liquid temperature. For any given mixing situation, evaporation rates will be proportional to the difference between liquid surface vapor pressure and the average vapor pressure in the mixing channel. Therefore, evaporation rates will increase very strongly as liquid surface temperature increases.
However, in an engine, there are strong practical reasons for wishing to minimize the amount of heat added to the mixture. First of all, low mixture temperatures help control detonation and maximize engine power. Secondly, hot surfaces tend to form undesirable deposits. The more rapid the mixing within the mixing section, the less heat or internal energy need be supplied to evaporate and mix the fuel (although the heat of vaporization must be supplied in any case) and the cooler the surfaces and mixtures can be. For all these reasons, mixing matters in the evaporation process. Fuel evaporation and fuel-air mixing are not the same thing, but the processes are coupled.
Mixing is a complex process, and is the interaction in space and time between molecular diffusion, turbulent diffusion, and the non-random convection of fluid elements along mean-flow stream lines. Most engineers are taught to feel that when a flow becomes significantly turbulent, it is no longer reasonable to attempt (even conceptually) to model the process in its details. It happens that this view, although widespread, is also wrong. Flow in many passages of practical interest can be characterized in terms of mean flow streamlines. These streamlines define flow patterns which occur because of basic internal and pressure equalization physical laws. Turbulence is superimposed on these mean flow streamlines, but the fraction of the kinetic energy in the flow which is in the form of the mean-flow pattern is often much larger than the fraction of the flow energy which is in the form of the very much smaller scale turbulent fluctuations. For certain practical sections, these mean-flow streamline patterns, or flow structures, are stable and reasonably predictable over quite wide ranges of massflow (and identically wide ranges of Reynolds number). These mean flow patterns can fold mixants together in an organized way, and greatly increase mixing rates over what would be possible with turbulence alone.
Mixing is the interaction of molecular diffusion (a random-walk process), turbulent diffusion (which is modelled as a random-walk process), and mean flow patterns (which involve non-random flow processes). It should be mathematically apparent that a random-walk process is much less efficient for traveling between two specified points than the properly chosen non-random process. Fundamentally for this reason, the properly chosen flow patterns can very greatly increase the rate of mixing in a mixing section, by doing the large-scale stretching and folding of the fluid elements in a non-random way. For example, the flow structure can systematically convect the least saturated air to the liquid surface for evaporation. The flow pattern can also systematically stretch out interfacial areas within the flow, increasing mixing area and reducing the mean distance over which the random-walk processes of turbulent and molecular diffusion must act. It turns out that properly chosen flow structures can in this way increase mixing rates by very large factors over mixing rates which would occur if the flow relations were purely random. Since there is little time and space available in an engine for fuel-air mixing, the more rapid mixing possible with structured turbulent flow is practically important.
Much background in producing desired flow structures has been worked out by the field of "fluidics," which uses flow relations to produce information-handling devices that operate because there are stable fluid flow modes in the flow elements of the fluid circuits. The physical laws which are most useful for producing predictable flow modes for flow structures are conservation of linear momentum, Bernoulli's equation which establishes well-defined relations between fluid velocities and fluid pressures, and the wall-attachment effect called "Coanda" effect. From conservation of linear momentum, conservation of angular momentum can be derived. It should be noted that the fluid mechanical laws, which fluidics teaches one to manipulate, continue to be valid when a flow becomes turbulent, therefore fluidics is very useful in permitting one to produce and understand flow structures under turbulent conditions.
The present invention uses fluidic fluid mechanics and mixing and heat transfer theory in the following way. Spark-fired engines are throttled, and the pressure drop across the throttle accelerates the flow in a near isentropic expansion, so that the flow velocity just downstream of the throttle is very often a significant fraction of sonic velocity (or sonic velocity itself). Just downstream of the throttle plate, the flow work across the throttle is stored in the fluid elements in the form of kinetic energy. These fluid elements have very significant linear momentum per unit mass. This flow momentum, properly utilized, is more than sufficient to produce a very strongly structured flow pattern downstream of the throttle plate. In current engines, the flow energy and momentum available just downstream of the air throttle is dissipated into turbulence and into unstable vortices. However, correctly designed deflectors can deflect this flow so that a high fraction of the isentropic velocity past the throttle plate is delivered in coherent form at high velocity into a channel. In the present invention, the outlet of this mixing channel is off-center with respect to the inlet point of the mixing channel by a distance R. If the flow velocity from the inlet point is resolved into vector components with respect to radial line R, including a vector component parallel to R, V.sub.r, and a velocity vector component perpendicular to the line R, V.sub.t, (or velocity tangential), then the fluid introduced into the channel will have angular momentum with respect to the center of the outlet. (Angular momentum is defined as MV.sub.t R). In the present invention, the channel peripheral walls are roughly concentric with respect to the mixer outlet. The flow from the deflectors from the throttle plate will progress until it interacts with the peripheral walls of the channel and it will lose some of its velocity by drag interactions with respect to this wall. However, if the passages are properly shaped, much of the momentum of the fluid will remain. Conservation of angular momentum is one of the most basic of physical laws. Therefore, it is relatively easy to design a mixture channel where the flow velocity from the deflectors is formed into a stable vortex flow pattern where the mean flow structure of the flow pattern is dominated by the physical relations of conservation of angular momentum. This flow characteristic will be discussed in more detail in the Detailed Description. However, it should be noted here that the vortex flow so established can be made to be a flow structure which is stable over a very wide range of Reynolds numbers (wide enough to cover the entire phase space of engine operation), and that the flow structure is one with very great advantages with respect to the mixing and evaporation functions which need to be served. First of all, the vortex flow pattern will serve strongly as a separator of the liquid fuel from the channel air flow, so that the fuel will deposit on the peripheral wall of the vortex chamber in a manner which should be easy to understand for those who understand cyclone scrubbers: The centrifugal forces in the vortex at the outside wall will be in the range of hundreds or thousands of G's. If this outside peripheral wall is heated, a very good heat transfer contact with the liquid fuel to be evaporated can be established. Secondly, the flow pattern is one wherein the air which is least saturated with fuel is the air which will be thrown to the outside of the vortex, so that the fuel is constantly exposed to the air which has the lowest vapor pressure of fuel in the chamber. Once the fuel is evaporated, the flow relations of the vortex (which is a turbulent flow with a pronounced mean flow streamline pattern) efficiently completes the mixing process. By the time an element of fuel air mixture reaches the central outlet of the vortex chamber it is substantially homogeneous all the way down to mean-free-path scales.
The operation of the vortex device is somewhat more advantageous than might at first appear. For example, under cold start conditions, the function of the vortex as a centrifugal separator of liquid droplets is most useful. During the start-up vapor leaves the vortex. If a rich mixture is delivered from the carburetor, an equilibrium splash-cloud of droplets around the outside vortex wall is quickly established. This splash-cloud has a very high surface area of liquid and very rapidly evaporates the light ends of the fuel into the air. The vortex at this time functions as an approximation of an equilibrium air distillation still and makes it possible to start the engine on a relatively lean mixture even during choke periods. Since the air fuel ratio coming out of the cold vortex can be much leaner than the mixture delivered to it, high CO and HC emissions are not necessary during the start-up process. For the same reason, the fuel penalties of cold start mixture enrichment need not be experienced since the cylinders need never see an over-rich mixture.
The mixer can be designed to warm up very quickly, and heating of the outside walls of the vortex channel is very advantageous because it permits the liquid surface to be evaporated to be at a much higher temperature than the mean temperature of the mixture in the vortex, so that the diffusion gradients driving the evaporation process can be made very large. In this way the peripheral walls can be operated so that evaporation rates are so fast that only a relatively small fraction of the peripheral wall needs to be wet at any time. This means that the mass of liquid fuel in the mixer at any time can be made very small, and therefore the time lag between fuel transport through the vortex and air transport through the vortex can be held to a very small value.
Under very low intake manifold vacuum (very high power demand) the operation of the vortex is beneficial, too. Under these conditions the liquid fuel evaporates and rapidly recondenses in the form of droplets well below one micron, so that the vortex functions as a smoke generator and mixer and the volumetric efficiency of the engine is increased because of the reduction in the mixture temperature delivered.
Experiment has shown that the present invention works well as an evaporator and mixer. Certain points are important with respect to its significance. First of all, it has been shown that the mixing device will produce cylinder-to-cylinder air fuel distribution which is so tight that cylinder-to-cylinder variation cannot be conveniently measured. This is fuel-air distribution much tighter than that attainable with fuel injection systems. Secondly, the mixing system has very rapid transient responses and the rapid transient responses very much reduce the necessary trade-off between low emissions and drivability. The microscale homogeneity from the system widens the lean limits of engine operation, making improvements in both efficiency and emissions possible. Also, the mixing device evaporates efficiently enough that it will tolerate gasolines having end boiling points significantly higher than those which are tolerable with current engine systems. This is an important issue, because widening the end point specification for gasoline significantly reduces refining costs. There are also indications (which will require more research to establish) that the vortex mixer will permit the use of napthalenic and other low hydrogen to carbon ratio hydrocarbons in motor fuel. If this proves to be possible it will greatly ease the problem of producing motor fuels from synthetic sources, such as tar sands, oil shale or coal. Government and industry supported research is now in progress concerning the vortex to test its ability to burn synthetic fuels, including alcohols and low hydrogen to carbon ratio fuels.
The vortex mixer will also permit the design of lighter, cheaper, and more efficient intake manifolds since these manifolds need not handle the complexities of two-phase flow. It should be mentioned that the mixing relations of the vortex mixer are such that it can be designed as a relatively low-drag device, permitting high peak power. The vortex mixer will also function to mix homogeneously EGR with the fuel and the air for even EGR distribution cylinder-to-cylinder. Because of these effects, it has been shown experimentally that the present invention vortex mixer simultaneously improves driveability, emissions, and overall fuel economy. The inventors wish to thank O. A. Uyehara, G. L. Borman, and P. S. Myers of the University of Wisconsin for many useful discussions during the development of the device.